No experience with it, but I read a thread recently on another forum about increasing the flame retardent properties of nomex flightsuits.
The following is a direct copy-paste from that:
"Borax and Boracilic Acid. You can get it from some chemists. Can't remember the proportions, but I can probably find out for you if you want.
It will flameproof your clothes (or re-flameproof your flying suit after washing). It really works! The only after affects of 1 minute's exposure to flame of some thin cotton that had been treated was that it was brittle. No burning though."
Not sure if that helps - I can only suggests some serious googling to find the proportions or someone who knows more!